Bluetooth (BT) is an example of an ad-hoc networking technology which is the wireless communication standard generally used when transferring information between two or more devices that are near to one another when speed is not an issue, including telephones, printers, modems and headsets. BT is well suited for low-bandwidth applications including transferring sound data with telephones (e.g., with a BT headset) or byte data with hand-held computers (transferring files) or from keyboards or mice. The BT Special Interest Group (SIG) specification may be used for these communications.
BT devices support high quality audio distribution. A conventional use case is the streaming of music content from a stereo music player to headphones or speakers. The audio data is compressed in a proper format for efficient use of the limited bandwidth. Another use case is the synchronizing of the picture and speech in a movie, for example.
A BT profile is a specification regarding an aspect of BT-based wireless communication between devices. The BT profile resides on top of the BT core specification and optionally other additional protocols. The Audio/Video Distribution Transport Protocol (AVDTP) comprises a signaling entity for negotiation of audio streaming parameters and a transport entity that handles streaming itself. AVDTP is used by the advanced audio distribution profile to stream music to stereo headsets over a Logical Link Control and Adaptation Layer Protocol (L2CAP) channel intended for the video distribution profile in the BT transmission.
The Advanced Audio Distribution Profile (A2DP), sometimes referred to as BT audio streaming, mandates usage of AVDTP. Most smartphones today are capable of connecting to A2DP devices and transmitting audio. A2DP defines a BT protocol and procedures that realizes distribution of audio content of high-quality audio in mono or stereo on asynchronous connection-less (ACL) channels. A2DP effectively transforms a BT connection into an invisible auxiliary audio cable. For example, using A2DP music can be streamed from a mobile phone, to a wireless headset, hearing aid/cochlear implant streamer, car audio, or streamed from a laptop/desktop to a wireless headset. Voice can be streamed from a microphone device to a recorder.
BT does not have any inherent way to synchronize audio between respective BT devices such as speakers. In the BT SIG specification, a synchronization code is embedded in the preamble of each BT packet to assist the receive device's timing synchronization process for each packet that is received. Unlike WiFi, due to lower relative power used for BT transmissions, BT has very limited broadcast capabilities. Moreover, broadcast in BT is not guaranteed to be received as there is no baseband acknowledgement (ACK) frame provided by the receiving devices. Synchronization can be performed at the device's host processor level which requires significant extra power and is also relatively complicated to implement. Current known audio synchronization solutions generally only support two audio devices configured in a pico-net topology, generally with the master as the audio transmitter and the slave as the sole audio receiver.